Sharp-eyed internet users spotted the animal in an official NASA photograph taken by the Curiosity Rover soon after it landed on Mars in 2012.
Conspiracy theorists believe the picture reveals a top-secret NASA experiment which the public were not informed about due to animal rights violations.
The UFO Sightings Daily website said: “It is really amazing and strange. I guess that’s why I love this one so much. It’s a cute rodent on Mars.”
The photographs show a very convincing squirrel-like object crouching between two boulder in a rocky expanse.
The object appears to have ears, eyes, a nose and front legs, as well as a long, thin body similar to that of a grey squirrel.
The website believes NASA’s fear of being sued is behind the lack of official confirmation of the squirrel’s presence on the fourth planet from the Sun.
It said: “A lot of people are emailing me saying that this squirrel was part of a NASA experiment to test how long it would live on the surface of Mars and I do believe this does sound like something they might do.
“Why would they not tell us about it? Because the squirrel would be expected to die eventually and that would get PETA to fight against them in a court of law.”
They believe another explanation for the lack of information is America’s desire to keep ahead of Russia and China in a new space race.
The website said: “They decided life on Mars was a secret worth keeping since they don’t want China or Russia to beat America to Mars.
“So they shut up about it and can take their time to Mars this way.”
It is not the first time a surprising object has beens potted on Mars. Earlier this month a huge skeleton was discovered, while last November conspiracy theorists claimed to have spotted a huge walled city.